1. Field of the Invention
This invention relates to a developing member such as a developing roller used in, e.g., electrophotographic image forming apparatus such as copying machines and laser beam printers, and also relates to a developing assembly and an image forming apparatus which make use of the developing member.
2. Description of the Related Art
As one of development processes used in electrophotographic apparatus such as copying machines, laser beam printers, and receiving sets of facsimile machines, a pressure development process is available in which a non-magnetic toner-component developer is used and the developer is made to adhere to an electrostatic latent image held on a photosensitive drum, to render the latent image visible. This pressure development process is widely used because it requires no magnetic material, facilitates manufacture of simple and compact apparatus, and also facilitates preparation of color developers.
In the electrophotographic image forming apparatus employing such a pressure development process, a photosensitive drum being rotated is uniformly electrostatically charged by means of a charging member Then, the photosensitive drum is exposed to laser beams to form electrostatic latent images on the photosensitive drum. Next, a developer is fed to the electrostatic latent images by means of a developing assembly, and the electrostatic latent images are developed to form toner images. Thereafter, the toner images are transferred onto a transfer material (recording material). Finally, the toner images on the transfer material are, e.g., heated, and thereby fixed to the transfer material.
Meanwhile, the surface of the photosensitive drum from which the toner images have been transferred is destaticized, and then cleaned to remove any developer remaining on the surface. Thus, the surface is brought into the state of stand-by for further image formation.
The above developing assembly is provided with a developer container holding therein the developer, a developing roller which is so disposed as to close the opening of the developer container and partly stand bare to the outside of the developer container, and a developer feed roller which coats the surface of the developing roller with the developer.
The developing assembly is further provided with a developing blade which adjusts to a more uniform thin layer the developer with which the surface of the developing roller is kept coated, and is so set up that, as the developing roller is rotated, the thin-layer developer can be transported to the part where the developing roller stands bare.
The thin-layer developer adheres to the electrostatic latent images formed on the photosensitive drum being rotated that is disposed opposingly to the developing roller at its part where it stands bare, and renders the electrostatic latent images visible to form toner images on the photosensitive drum.
The developing roller used in such a developing assembly has a stated electrical resistance value, and is required not to change in electrical resistance and also required not to contaminate the photosensitive drum.
As a developing roller meeting such requirements, Japanese Patent No. 3186541 (Patent Registration 3186541) discloses a proposal of a conductive member making use of a urethane containing as a polyol component a poly(ether-polyol) in which ethylene oxide and propylene oxide have been added at random to glycerol.
Japanese Patent Application Laid-open No. 2003-167398 discloses that a urethane resin is preferable as a material for forming a surface layer of a conductive member used in the developing assembly of the electrophotographic image forming apparatus, but on the other hand has disadvantageous properties that it has a high tackiness and also may cause a large friction. Then, it discloses that adding a polysiloxane component to the urethane resin enables solution of the above problems on tackiness and frictional properties.
Now, the developer feed roller and the developing roller are, at the time of operation, mutually rotated and in contact with each other. Also, it follows that these continue to come into contact with each other at the same position over a long period time when the electrophotographic apparatus is continuously kept to stand unoperated over a long period time. If thereafter the process of forming electrophotographic images is carried out, line-shaped image non-uniformity (hereinafter “banding”) may come about in the electrophotographic images formed. The banding may be remarkable especially in halftone images, and tends to occur as a result of leaving in an environment of high temperature and high humidity (e.g., temperature 40° C./humidity 95% RH) over a long period time.
The cause of occurrence of the banding is not necessarily clear. The present inventors presume it as stated below. That is, any components exuding out of the developer feed roller may adhere to the developing roller surface, where it follows that the developing roller surface differs in chemical composition from place to place. If electrophotographic images are formed using such a developing roller, the toner may come different in triboelectric charge quantity between areas to which the exuded components have adhered and areas not adhered, of the developing roller surface. In other words, the triboelectric charge quantity comes non-uniform. This is presumed to be one of the causes of occurrence of the banding.